Predicting the distribution and stability of photoactive defect centers in nanodiamond biomarkers

Recent observations of photoactive silicon-vacancy (Si-V) color centers in diamond nanoparticles less than 10 nm in size has prompted interest in this material for optical labeling in biomedical applications. In order to be useful in such situations, these Si-V defects need to be in sufficient concentrations, and must be stable with respect to diffusion at room and body temperature. In this paper, density functional tight binding simulations are used to systematically examine the configuration, distribution and thermodynamic stability of the neutral and charged Si and Si-V centers in representative diamond nanocrystals. The results indicate that the stability of neutral Si-V is superior to other Si-related defects when the particle surfaces are suitability passivated, in agreement with PL measurements. Based on these results we show the size dependence, and thermal stability of this defect make fluorescent nanodiamond an ideal candidate for biomarkers.